In this paper, pen computing, i.e. the use of computers and applications in which the pen is the main input device, are described from four different viewpoints. Firstly a brief overview of the hardware developments in pen systems is given, leading to the conclusion that the technological developments in this area have not led to the expected user acceptance of pen computing. The reasons underlying this market failure are explored. Problems of pen-user interface design are then described and existing and new applications are summarised. The handwriting process and product are discussed and, finally, automatic recognition methodologies are considered. Four basic factors determining handwriting variation and variability are identified. A handwriting recognition approach using segmentation into velocity-based strokes is considered in somewhat more detail.